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82 SMT007 MAGAZINE I SEPTEMBER 2024 qualified for use in actual applications. Electro- migration research can be better conducted on planar-solder joints 9 . Planar solder joints are easy to fabricate in a typically equipped metal- lurgical laboratory in a matter of a day or two and electromigration can be observed on a real time basis if the test specimen is mounted on a hot stage in an electron microscope. Figure 10 compares the geometry of a BGA to a planar solder joint and Figure 11 is a top-down view of a planar test specimen with five solder joints of various lengths in the range 0.18–0.41 mm. e details of the planar solder joint fabrica- tion have been presented earlier 10–13 . A detailed study comparing the eutectic Sn-Bi electromi- gration in the planar and the BTC geometry concluded that the electromigration behav- ior was similar in the two cases as shown in the Arrhenius plots of Figures 12 and 14. ere- fore, planar solder joints can be conveniently used to study the electromigration propensity of solder metallurgies to obtain a short list of alloys for further testing in actual applications. Conclusions Phase diagrams are very useful in predict- ing solder alloy behavior once one recognizes that they are for equilibrium condition and under non-equilibrium cooling conditions, the microstructures are not quite what the phase diagram predicts. e high homologous tem- peratures of Sn-Bi alloys is the reason they tend toward thermodynamically equilibrium micro- structures quite quickly even at temperatures as low as room temperature. e volume frac- tion of the Bi-rich phase in a cast Sn-Bi alloy will always increase when aged at room tem- perature, though observations of the surface of The New Chapter Feature Column by Hannah Grace, TEXAS INSTRUMENTS Figure 10: a) BGA solder joint between a PCB and a component; b) planar solder joint on a PCB. Figure 11: Five planar solder joints of various length in series. Source: IBM Corporation, Presented at SMTA International 2023.